Kipp&Zonen CGR 4 Pyrgeometers - Manual (Page 17)

The solar blind filter is opaque to radiation below 4.5 µm, known as the cut-on wavelength. Currently
most pyrgeometers have their cut-on at a lower wavelength. Problems may occur in the case of clear
sunny days with low humidity. In the solar spectrum between 2.5 and 4.5 µm there can still be an
amount of infrared solar radiation up to 10 W/m², which should not be included in the measurement.
This unwanted fraction is blocked in the CGR 4 by the filter coating.
CGR 4 uses a specially designed pure silicon dome. Although the dome is not hemispherical, CGR 4 has
a 180° field of view with good cosine response. A big advantage of the meniscus shaped dome over the
typical hemispherical dome is the ability to deposit more uniform coatings on the window surface.
Deposition of a uniform filter coating on a strongly curved surface is very difficult and unpredictable
process. To avoid these problems Kipp & Zonen developed a dome with excellent optical quality due to
the optimised shape and coating uniformity.
The solar radiation absorbed by the window is conducted away very effectively by a unique construction
in the CGR 4. Even in full sunlight the window heating effect is very low compared to that of other
pyrgeometers on the market. This allows accurate daytime measurements without the need for a
shading disk. It also eliminates the need for window heating compensation by using the correction
formula and window temperature sensors.
3.2. Detector
The thermopile sensing element is made up of a large number of thermocouple junction pairs
connected electrically in series. The absorption of thermal radiation by one of the thermocouple
junctions, called the active (or ‘hot’) junction, increases its temperature. The differential temperature
between the active junction and a reference (‘cold’) junction kept at a fixed temperature produces an
electromotive force directly proportional to the differential temperature created. This is a thermoelectric
effect. The sensitivity of a pyrgeometer depends on the individual physical properties of the thermopile
and construction. The sensitivity of each thermopile is unique and therefore each radiometer has
unique calibration factor, even with the same radiometer model.
On the top surface of the sensor a black paint is deposited which has a very rough structure containing
many micro-cavities that effectively ‘trap’ more than 95% of the incident radiation in a broad spectral
range. Furthermore, the spectral selectivity is less than 3%. This means that within the spectral range
of the pyrgeometer, the absorption for each wavelength is equal to within 3%. The black painted
sensing element forms the detector. Considering the long-term stability of the instrument, the black
paint is one of the most crucial and delicate parts of the pyrgeometer. Kipp & Zonen black paint gives
the best possible stability over a long period of time under all meteorological circumstances.
3.3. Temperature sensor
The housing temperature sensor is a crucial part of a pyrgeometer and is needed to calculate the
downward long-wave radiation component. The body temperature sensor represents the ‘absolute’
temperature of the detector surface and therefore it is mounted close to the cold junctions of the
detector. A housing temperature sensor is fitted as standard to the CGR pyrgeometers.
Page 17 CGR 4 Manual